The present invention relates to the field of rapid prototyping, and particularly to methods of achieving surface smoothness in prototype objects made by layered manufacturing.
Production and testing of prototype objects is a commonly used step in developing new products, machines and processes in a wide range of industries. A variety of layered manufacturing methods for forming three-dimensional prototypes are known, which develop prototype objects cheaply and quickly from a geometric computer model under computer control. These rapid prototyping methods generally slice or divide a digital representation of a desired object (computer aided design (CAD)) into horizontal layers, then build the object layer-by-layer by repetitive application of materials. Exemplary rapid prototyping techniques include layered deposition modeling, selective laser sintering and stereolithographic processes.
One example of layered deposition modeling is a fused deposition modeling technique performed by Stratasys® FDM® modeling machines. Fused deposition modeling builds up three-dimensional objects by extruding solidifiable modeling material from an extrusion head in a predetermined pattern, layer-by-layer, based upon design data corresponding to the particular shape of each object layer. Examples of extrusion-based apparatus and methods for making three-dimensional objects are described in Crump U.S. Pat. No. 5,121,329, Crump U.S. Pat. No. 5,340,433, Danforth et al. U.S. Pat. No. 5,738,817, Batchelder et al. U.S. Pat. No. 5,764,521 and Dahlin et al. U.S. Pat. No. 6,022,207, all of which are assigned to Stratasys, Inc., the assignee of the present invention.
In the Stratasys® FDM® modeling machines of the current art, modeling material is typically loaded into the machine as a flexible filament wound on a supply reel, such as disclosed in U.S. Pat. No. 5,121,329. A solidifiable material which adheres to the previous layer with an adequate bond upon solidification and which can be supplied as a flexible filament is used as the modeling material Motor-driven feed rollers advance the strand of filament into a liquifier carried by an extrusion head. Inside the liquifier, the filament is heated to a flowable temperature. Flowable modeling material is forced out of a nozzle on the far end of the liquifier, and deposited from the liquifier onto a base. The flow rate of the material extruded from the nozzle is a function of the rate at which the filament is advanced to the extrusion head. A controller controls movement of the extrusion head in a horizontal x, y plane, controls movement of the base in a vertical z-direction, and controls the rate at which the feed rollers advance filament. By controlling these processing variables in synchrony, the modeling material is deposited in “beads” layer-by-layer along tool paths defined from the CAD model. The material being extruded fuses to previously deposited material and solidifies to form a three-dimensional object resembling the CAD model.
The surfaces of objects developed from layered manufacturing techniques of the current art are textured or striated due to their layered formation. Curved and angled surfaces generally have a “stair step” appearance, caused by layering of cross-sectional shapes which have square edge profiles. The stair-stepping effect is more pronounced as layer thickness increases. Although the stair-stepping does not effect the strength of the object, it does detract aesthetically.
Surface roughness of objects created by layered manufacturing techniques also arises from errors in the build process. For example, in the fused deposition modeling systems of the current art, errors can arise due in part to inconsistent extrusion flow rates. Errors particularly occur at start points and end points of the tool path, for instance, at the location of a “seam” (i.e., the start and end point of a closed-loop tool path). These errors can cause undesired inconsistencies in the shape of the resulting model.
Rapid prototyping of three-dimensional objects includes not only the production of prototype parts, but also may include the production of molds. Exemplary uses of molds created with rapid prototyping include forming molds used to create injection molding tool inserts such as described in U.S. Pat. No. 5,189,781, and forming fugitive molds for green ceramic pieces prior to sintering such as described in U.S. Pat. Nos. 5,824,250 and 5,976,457.
The current art teaches manually trimming, machining or grinding objects formed by layered manufacturing to remove excess material. Buffing with cloths, sand paper or solution-born abrasives are current methods of smoothing or polishing the objects. For example, Hull et al. U.S. Pat. No. 5,059,359, Methods and Apparatus for Producing Three-dimensional Objects by Stereolithography, describes their prototypes as “perfect for smoothing by sanding to yield the right-sized part”. The need for hand-finishing of models made from additive process techniques is also recognized in U.S. Pat. No. 6,021,358, which utilizes subtractive modeling techniques to attain smooth models. There is a need in rapid prototyping systems of an expedient and relatively inexpensive method of post-processing layered manufacturing prototype objects.
A previously developed technique used in manufacturing of plastics involves the use of chemical vapors or liquids to smooth by reflowing the surface of the plastic, termed solvent polishing. Solvent polishing was developed in the plastics industry over twenty years ago for the purpose of developing a smooth level and/or high gloss coating or surface without needing to exercise extreme care in the application or manufacturing of the items. For example, U.S. Pat. No. 3,437,727 discloses a method using chemical vapors for refinishing telephones that were returned to the telephone company as a method of recycling them.
There are two standard methods for solvent polishing articles. The first is to immerse the entire plastic article in a bath of liquid plastic solvent for a period of time based on the solvent and type of plastic involved. This allows the solvent to penetrate the outer layer of the plastic, thereby causing it to reflow. Reflowing causes the outer surfaces of the plastic article to become smooth and/or shiny.
The second method of solvent polishing is the exposure of the plastic article to vaporized solvent. A handheld vaporizer as shown in U.S. Pat. No. 4,260,873 may be used to expose the plastic object. Alternatively, the object can be placed into a chamber filled with a vaporized solvent, generated from a heated bath below, as in U.S. Pat. No. 3,737,499. An advantage of the vaporizing chamber is that the solvent is contained and can be recycled, thereby minimizing potential environmental pollution.
The use of hot solvent vapors to melt or plasticize the surface of the substrate has been used in the circuit board manufacturing area to facilitate the transfer of printed circuits, as disclosed, for example, in U.S. Pat. No. 4,976,813. Another example is disclosed in U.S. Pat. No. 4,594,311, where solvent vapor is used to treat the non-imaged areas of the plastic base material which holds a printed circuit board in order to further enhance the printed pattern and secure it more strongly to the surface. In U.S. Pat. No. 5,045,141, a substrate, typically a circuit board, may be treated to facilitate transfer of the printed circuit to it.
Solvent polishing using liquid or vapors is also commonly used as a degreasing or cleaning step in manufacturing processes, especially prior to painting.
Despite the need in rapid prototyping for an expedient and inexpensive surface finishing technique, Applicant is unaware of any teaching or suggestion in the prior art to use a vapor polishing technique for the smoothing of objects formed by layered manufacturing rapid prototyping techniques.